Default Sales Tax

You can establish a default Sales Tax for each customer and the default Sales Tax will automatically default in Invoice/Credit Memo, Estimate and Sales Order and Cash Sale for the customer's documents that are assigned to the customer. 

Document default Sales Tax logic is as follows:

To use a different sales tax rate for the document, choose a different tax from the Default Tax drop down list.

Although it is not required that you enter a default Sales Tax, it makes data entry faster.  Also, it can be overridden in those screens that use this default.